Full-duplex communications, in which a transmitter and a receiver of a transceiver operate simultaneously on the same frequency band, is drawing significant interest for emerging 5G communication networks due to its potential to double network capacity compared to half-duplex communications. However, one of the biggest challenges from an implementation perspective is the antenna interface.
One way in which an antenna interface for a full-duplex transceiver can be implemented is using a non-reciprocal circulator. Reciprocity in electronics is a fundamental property of linear systems and materials described by symmetric and time-independent permittivity and permeability tensors. Non-reciprocity in a circulator causes signals to travel in only one direction through the circulator. This unidirectional signal flow enables full-duplex wireless communications because signals from a transmitter can be only directed toward an antenna (and not the receiver) and received signals at the antenna can be only directed toward the receiver (and not the transmitter). Thus, a circulator allows transmitter-to-antenna signal transmission and antenna-to-receiver signal transmission with very low loss, and provides isolation to the receiver from the transmitter, thus protecting the receiver from the transmitter's interference.
Conventionally, non-reciprocal circulators have been implemented using ferrite materials, which are materials that lose their reciprocity under the application of an external magnetic field. However, ferrite materials cannot be integrated into CMOS IC technology. Furthermore, the need for an external magnet renders ferrite-based circulators bulky and expensive.
Accordingly, new mechanisms for implementing circulators and full duplex wireless transceivers are desirable.